Generally,
(1) you look in any solid-state physics textbook that discusses the LJ potential,
(2) you search in a scientific articles search engine for articles.

I guess you want to simulate liquids or noble gases, or you are more interested in the dynamics of atoms rather than the material itself. Other than that, there's no point using LJ potential.

If (1) and (2) turns out a blank for the molecule you want to simulate, then you can do a simple potential parametrization depending on what you want to simulate eg. sigma = nearest neighbour distance of your molecule, epsilon is energy per atom for dimer .